git.sur5r.net Git - cc65/blob - libsrc/runtime/umul16x16r32.s

   1 ;
   2 ; Ullrich von Bassewitz, 2010-11-03
   3 ;
   4 ; CC65 runtime: 16x16 => 32 unsigned multiplication
   5 ;
   6
   7         .export         umul16x16r32, umul16x16r32m
   8
   9         .include        "zeropage.inc"
  10
  11
  12 ;---------------------------------------------------------------------------
  13 ; 16x16 => 32 unsigned multiplication routine.
  14 ;
  15 ;   lhs         rhs           result          result also in
  16 ; -------------------------------------------------------------
  17 ;   ptr1        ax            ax:sreg          ptr1:sreg
  18 ;
  19
  20 umul16x16r32:
  21         sta     ptr3
  22         stx     ptr3+1
  23
  24 umul16x16r32m:
  25         lda     #0
  26         sta     sreg+1
  27         ldy     #16             ; Number of bits
  28
  29         lsr     ptr1+1
  30         ror     ptr1            ; Get first bit into carry
  31 @L0:    bcc     @L1
  32
  33         clc
  34         adc     ptr3
  35         pha
  36         lda     ptr3+1
  37         adc     sreg+1
  38         sta     sreg+1
  39         pla
  40
  41 @L1:    ror     sreg+1
  42         ror     a
  43         ror     ptr1+1
  44         ror     ptr1
  45         dey
  46         bne     @L0
  47
  48         sta     sreg            ; Save byte 3
  49         lda     ptr1            ; Load the result
  50         ldx     ptr1+1
  51         rts                     ; Done
  52